This proposal represents the continuation and natural extension of our studies in which we have provided additional support for our original discovery that identified a prominent phosphoprotein substrate for protein kinase C (PKC) in brain (MARCKS; Myristoylated Alanine Rich C Kinase Substrate) as a molecular target for the action of chronic lithium in hippocampus. We propose to continue our investigations of the action of chronic lithium on MARCKS regulation using immortalized hippocampal cells which have been subcloned during the previous grant period and are studied under inositol-limiting conditions specifically designed to provide a physiologically relevant model. Using specific monoclonal antibodies and recombinant DNA strategies developed in our laboratory we will determine in this cell model the sequence of transcriptional and posttranscriptional intracellular events mediating the lithium-induced down-regulation of MARCKS expression. Preliminary studies in our laboratory have recently provided evidence in support of a PKC-mediated destabilization of MARCKS mRNA in these cells. Our proposal seeks to examine the uniqueness of this action of lithium in the brain by: (1) concomitantly examining PKC phosphoprotein substrates (MRP/GAP-43) in brain which share some of the properties of the MARCKS protein, and (2) pursuing our preliminary findings that valproate (which appears to possess similar clinical properties to lithium may share a common mechanism of action vis a vis regulation of MARCKS expression. In addition we will further confirm the physiological relevance of our findings in both primary hippocampal cells as well as a rat model where we already have preliminary data of a heightened expression of MARCKS in selective regions of normal hippocampus using in situ hybridization. In concert with our participation in the Stanley Foundation Neuropathology Consortium, and NIMH Brain Bank, we will be extending our studies to postmortem brain tissue from patients with well documented and characterized manic-depressive illness (MDI). We will be able to examine for the first time the relationship between lithium and MARCKS regulation in human brain and potentially establish a role for MARCKS in the pathobiology of MDI. Our discovery of the lithium-induced regulation of the expression of this phosphoprotein, stemming from its action in receptor-coupled PI signaling and PKC activation, offers us a window through which we can begin to understand the downstream molecular impact of long-term lithium on neuroplastic events involving cytoskeletal remodeling and modulation of signal transduction, which may be associated with its therapeutic efficacy in this disorder. Moreover, we anticipate these studies will also ultimately enable us to initiate rational drug discovery efforts for the development of the next generation of mood stabilizers.